I had the exact same idea years ago. Eventually I found there are some shortage about it so I ditched the idea:

- How to verify people achieved their goal (this website introduces "supervisor" but I'm not sure that's gonna work well)

- You gain nothing from achieving goals, v.s. you lose money if you didn't

- The gain (goal achieved, which can be done without losing money) is much less than lose (lose real money), so no incentive for people to use this service.

This website will face the same challenge.

I'm happy to see there are someone out there having the exact same idea, I hope they can do well sincerely.